The miniaturization of electronics has led to devices that are faster, cheaper, and more versatile with every new generation. Building micromechanical devices offers similar benefits. For example, resonant frequencies increase as dimensions are reduced, allowing high bandwidth accelerometers and pressure sensors to be built. Miniature devices integrating actuators, limbs, sensors, computing and energy supplies offer, among other things, the potential to access microscopic spaces, with major implications for medicine. In large scale devices, nature offers many examples of what can be achieved by fabricating macroscopic systems with nanoscopic precision, including ourselves.
In order to realize the potential offered by micro and nanotechnology, it is necessary to manipulate material on micrometer and nanometer scales. Important characteristics of fabrication technologies include (1) spatial resolution, (2) achievable geometry, (3) available materials, (4) fabrication rate and (5) cost. Spatial resolution is defined by the dimensions of the smallest feature that can be produced by a given fabrication method. Achievable geometry refers to the range of shapes a method is capable of producing. The ability to incorporate a wide range of materials is particularly important because microdevices, like macrosystems, in general require actuators, energy delivery and storage, mechanical elements, circuits, and sensors. These functions are unlikely to be optimized using a limited range of materials. The difficulty in assembling and fastening pars on micrometer scales make geometric versatility and wide range of materials particularly desirable properties of a fabrication process.
Traditional milling, welding and fastening technologies do not have the spatial resolution required to generate microdevices. Integrated circuit technology is increasingly being applied to fabricate mechanical, electro-mechanical and electro-opto-mechanical devices on millimeter and micrometer scales, including miniaturized accelerometers and pressure sensors. However, the nearly two-dimensional regime of this technology restricts applications and performance. In general, the low aspect ratio (height to width) structures of uniform thickness produced do not optimize functionality. While multiple layers have been employed to add thickness and features in a third dimension, the long development times, low yields and high costs involved restrict the number of layers which are practical. Furthermore, the materials used are primarily silicon-based.
Lithography-related techniques such as LIGA (from the German for lithography, electroplating and molding) allow aspect ratios to be increased substantially (see for example U.S. Pat. No. 5,162,078 in the name of P. Bley et. al. issued Nov. 10 1992). In these techniques two dimensional mask patterns are etched into resist layers that can be more than 300 micrometers thick. The resist layers are often used as molds. While lithography-related techniques produce high aspect ratio structures with high lateral resolution, these structures are of uniform thickness, or essentially two dimensional structures with added thickness.
Several three dimensional microfabrication technologies are under development, the most notable being focussed beam excimer laser machining, stereo lithography and laser-assisted chemical vapour deposition.
A three dimensional fabrication technology is defined as one that can be employed to generate objects of virtually arbitrary geometry. In general, achievable geometries are limited only by the requirements of material continuity with a supporting substrate and mechanical stability of the objects being constructed. Three dimensional fabrication technologies are capable of generating objects such as helically coiled springs and hollow spheres. Two classes of three dimensional fabrication technologies exist, namely those in which material is removed from an existing body, as in milling and sculpting, and those in which material is added to build up a structure, as in brick laying and stereo lithography. Material addition methods generally require few or no assembly steps. For example, the fabrication of a hollow sphere can be achieved in one process, without assembly, by the method of this invention, whereas with a machining process at least one assembly step is necessary. The minimization of assembly steps is especially important in microfabrication because manipulation of parts is particularly challenging. The ability to combine material removal and addition in one technology is very valuable because large numbers of part geometries can be constructed and modified, either by addition or removal of material.
Fabrication methods wherein patterns of relatively uniform depth are generated on a surface are not considered three dimensional. These include methods of patterning uneven and contoured surfaces.
Focussed excimer laser beams have been used to ablate a wide range of materials, including polymers, ceramics and metals (see for example J. M. Hagerhorst et. al. "Focussed Excimer Laser Beams: A Technique for Micropatterning", SPIE, vol. 52, pp.299-304, 1992.). Resolution of focussed beams is diffraction limited to approximately .apprxeq.0.2 micrometers. However, there is a tradeoff between depth of focus and resolution since high resolution requires high numerical aperture beams. Hence high aspect ratio is only practical with micrometer or larger features. Material removal technologies, such as excimer machining, are in general limited in their geometrical versatility, in part because they access through the outer surface of an existing substance.
In U.S. Pat. No. 4,929,402, issued May 29, 1990, C. H. Hull teaches a stereolithography method which has been employed by Ikuta et. al. to build microstructures ("Three Dimensional Micro Integrated Fluid Systems Fabricated by Stereo Lithography", Proceedings--IEEE Micro Electro Mechanical Systems, pp. 1-6, 1994.). In-stereo lithography a beam of radiant energy interacts with a photo-sensitive polymer, causing it to harden. Controlled volumetric irradiation of the polymer allows structures of virtually arbitrary geometry to be fabricated. Materials are limited to a selection of photosensitive polymers, however. Resolution is restricted by diffraction limits and viscous forces from the unhardened polymer on the hardened material. The best resolution achieved to date is about 10 micrometers.
M.Boman et. al. describe a microfabrication method employing laser assisted chemical vapour deposition (LCVD) to fabricate three dimensional structures ("Helical Microstructures Grown by Laser Assisted Chemical Vapour Deposition", Proceedings--IEEE Micro Electro Mechanical Systems, pp. 162-167, 1992.). T. M. Bloomstein et. al. describe a similar method for depositing and etching material in U.S. Pat. No. 5,389,196, issued Feb. 14, 1995. A focussed photon, ion or electron beam interacts with a substrate surface in a vapour atmosphere, inducing a local chemical reaction, and resulting in material being deposited from the vapour or etched from the substrate. Reported resolutions using photon beams are .ltoreq.10 micrometers, limited by diffraction and thermal conduction at the substrate surface. Electron beams have enabled 0.2 micrometer resolution, but at the expense of deposition rate, which is on the order of 0.02 micrometers per second (see W. H. Brunger et. al., "E-beam Induced Fabrication of Microstructures", Proceedings--IEEE Micro Electro Mechanical Systems, pp. 168-170, 1992). Photon beam induced deposition rates of several micrometers per second are achieved in building three dimensional structures. In the deposition process, materials are restricted to those that can be grown on a substrate from vapour phase via a chemical reaction. Common materials are tungsten, boron and silicon.
Electrochemical deposition and Etching
A wide range of materials can be electrodeposited, including many metals and alloys, some polymers (such as polyaniline and polypyrrole), and semiconductors (such as cadmium chalcogenides). Electrochemical etching enables many of the same materials to be machined as well as others such as gallium arsenide. However, in conventional electroplating deposition occurs indiscriminately on a substrate, forming thin films rather than three dimensional structures. Rates of deposition are generally slow (e.g. about 100 micrometers per hour). Electroforming is used to build three dimensional structures. There is still the difficulty of fabricating the mold however.
Three basic approaches have been taken to create lateral features in deposits. The first makes use of lithography to selectively etch features in a deposited film. This method is used to make electrical connections for integrated circuits, for example. The aspect ratios achieved are particularly low due to etching undercut. Two other approaches involve either locally enhancing deposition rate by local heating, or placing a small electrode near the substrate to localize the reaction.
In laser enhanced electroplating and etching, laser light is focussed on a substrate, heating a local region, as taught by U.S. Pat. No. 4,497,692 (Gelchinski et. al., Feb. 5, 1985) U.S. Pat. No. 4,432,855 (L. T. Romankiw, issued Feb. 21, 1984), and U.S. Pat. No. 4,497,692 (D. R. Vigliotti et. al., issued Feb. 5, 1985) The heating locally enhances deposition and etching rates by as much as three to four orders of magnitude. Jet-plating and etching, in which a jet of plating or etching solution locally impinges on a substrate, is combined with laser enhanced electroplating, as described in U.S. Pat. No. 4,349,583, to improve deposit and etch properties, and to further enhance reaction rate. Laser enhanced electroplating (etching) produces two dimensional patterns deposited (etched) by scanning the beam parallel to the substrate surface. Resolution is again limited by diffraction as well as the substrate thermal conductivity. Reported resolutions are .gtoreq.2 .mu.m. Deposition rates, however, are high (about 10 micrometers per second)
Lin et. al. describe a method whereby placing a sharp-tipped electrode (scanning tunneling microscope tip) close to a substrate surface in a conducting solution, and applying a selected potential between the tip and the substrate, local etching is induced ("High Resolution Photochemical Etching of n-GaAs with the Scanning Electrochemical and Tunneling Microscope", Journal of the Electrochemical Society", vol. 134, pp.1038-1039, 1987). They use an approximately 0.1 micrometer diameter electrode in close proximity (&lt;1 micrometer) to a GaAs substrate to etch 0.3 micrometer wide lines. The GaAs substrate is uniformly irradiated by alight source to make it conductive. An electrochemical reaction is induced on the substrate whose spatial distribution is a function of the surface current distribution, which in turn depends on the applied potential, and the tip to substrate distance. The tip was scanned parallel to the substrate to form lines. No attempt was made to build a three dimensional object.
U.S. Pat. No. 4,968,390 describes a method to electrochemically deposit conducting substances on the surfaces of conducting objects (Bard et.al., issued Nov. 6, 1990). The invention involves coating a substrate with a thin, ionically conducting film containing the reactant to be deposited, and scanning laterally across the film to deposit and etch lines of widths as small as .apprxeq.0.2 micrometers, where the feature size is limited by the sharpness of the tip. Deposition and etching in three dimensions is not possible using this method since the solid electrolyte effectively fixes the tip to substrate spacing.
Schneir et. al. describe a similar method wherein an electrode is placed 1 micrometer from a gold substrate in a liquid plating solution and a potential is applied across the gap ("Creating and Observing Surface Features with a Scanning Tunneling Microscope", SPIE vol. 897, pp. 16-19, 1988). Positioning of the tip relative to the substrate is achieved by first finding the substrate surface using the tip in tunneling mode. In their method the tip and the substrate are immersed in an electroplating solution. The applied potential induces electrodeposition at the surface. The tip was moved back and forth in a line parallel to the gold substrate to produce a line of electrodeposited gold about 0.3 micrometers wide. There is no description or discussion of using this technique to build three dimensional objects.
It is the object of the invention to provide a process for producing three dimensional structures with sub-micrometer spatial resolution employing a range of materials selected from metals, polymers, and semi-conductors that will attempt to overcome the lack of fabrication methods capable of producing three dimensional objects from a wide range of materials and with sub-micrometer spatial resolution.
Further objects and advantages will become apparent from a consideration of the drawings and ensuing descriptions.